Ferromagnetic heating shares the advantages of other types of interstitial hyperthermia that the heating is delivered from within the tumor with more comprehensive heating of the entire tumor volume and greater ease of thermometry. Another advantage unique to ferromagnetic hyperthermia is that the nickel and copper alloy converts energy from a surrounding EM field into heat energy, but only up to a preset level (e.g., 45 degree C). This heat diffuses in a predictable fashion throughout normal rabbit muscle tissues as shown in preliminary experiments conducted in our laboratory. We also showed that it is feasible to combine radioactive iodine (I-125) seeds with ferromagnetic thermoseeds so that ionizing radiation and interstitial hyperthermia can be delivered simultaneously. Our goal is to optimize the combination of these two treatment modalities by studying the effect of several physical and radiobiological factors on the thermal enhancement ratio (TER) of ferromagnetic hyperthermia with I-125 brachytherapy for the local control of transplanted neoplasms in rabbits. Variables to be examined will be those with the greatest potential for clinical impact: spacing I-125 and thermoseeds within the implant, as well as timing, dose and fractionation of each treatment modality. We will also compare the efficacy of concomitant versus sequential hyperthermia and low- dose rate brachytherapy, in order to test our hypothesis that concomitant therapy with both modalities given simultaneously will lead to the greater therapeutic benefit than conventional (sequential) therapy.